1. Field of the Invention
The present invention relates to a control device for a machine tool. More specifically, the invention relates to a control device for a machine tool including a rotary indexing device composed of a rotary table on which a work is mounted, a clamping mechanism for holding an angular position of the rotary table, and a drive motor for driving the rotary table.
2. Description of the Related Art
In a machine tool including a rotary indexing device composed of a rotary table, a clamping mechanism for holding the rotary table, and a drive motor for driving the rotary table, a work is mounted on and fixed to the rotary table, the rotary table is rotated by the drive motor to position the rotary table to a predetermined angular position, the clamping mechanism is driven to clamp the table at the positioned position and then, the work is subjected to machining.
As the rotary indexing devices, one described in JP 10-29125 A is known. In the machine tool including such a rotary indexing device, a tool is brought into contact with a work mounted on and fixed to a rotary table to machine the work, but when the work is subjected to the machining, a load is applied to the work, and torque for rotating the rotary table is generated by this load. If the generated torque is great, the rotary table rotates against rigidity of the clamping mechanism in some cases. Hereinafter, position-holding force caused by rigidity of clamping mechanism for holding rotary table at predetermined rotation position is called clamping force. If the rotary table rotates from the positioned rotation position, the drive motor which drives the rotary table also rotates. Since the position of the drive motor is controlled, control for increasing output torque to return the rotary table to the positioned rotation position is performed, and the rotary table is held at the positioned rotation position by this position control.
If the rotary table is rotated by a machining load or the like and the rotation position deviates from the positioned rotation position (target rotation position), the drive motor outputs large output torque to resolve the deviation. If this state continues, the drive motor is brought into an excessive load state. As a method for preventing the drive motor from being brought into the excessive load state, JP 2008-134861 A describes a method in which if torque command sent to a drive motor becomes equal to or higher than a set value, the drive motor is prevented from being brought into the excessive load state by changing control of a speed controller from PI control (proportional integral control) to P control (proportional control), or by changing a gain of the speed controller, or by limiting a torque command value to a torque limit value or smaller by torque limit unit to drive a drive motor and suppress the output torque to a small value.
FIG. 5 shows a block diagram of a motor drive control system for controlling a position and speed of a servo-motor. A drive motor for driving the rotary table is also composed of a servo-motor, and a motor controller 24 controls a position and speed of the rotary table. A position controller 100 obtains position deviation by a position command and a position feedback signal from a position/speed detector 103 provided in the servo-motor, multiplies the position deviation by a position gain, and outputs the speed command to the speed controller. A speed controller 101 obtains speed deviation by a speed command and a speed feedback signal from the position/speed detector 103, performs proportional integral control (PI control), and outputs a torque command (current command) to a current controller 102. The current controller 102 performs current loop control by this torque command (current command) and a current feedback signal from a current detector (not shown) provided in a servo-amplifier, and drives and controls the servo-motor M through a servo-amplifier 25.
The drive motor for driving the rotary table is also composed of a servo-motor, and this motor controller 24 controls a position and speed of the drive motor. Hence, if the rotary table rotates and a position thereof is deviated from the positioned rotation position, position deviation at the position controller 100 increases, a speed command and a torque command also increase, the drive motor outputs large torque, control is performed to return the rotary table to a positioned angle (target position), control is performed such that the position deviation becomes 0, and position control is performed such that the rotary table is held at a positioned rotation position (target position). However, if a machining load is large, deviation of a rotation angle position cannot be reduced in some cases. The drive motor which drives the rotary table controls a position such that the rotary table is held at the positioned rotation position (target position) while the machine tool machines a work. Therefore, the rotary table is rotated by a machining load, rotation position is deviated from the positioned rotation position and position deviation increases. A speed command obtained by multiplying the position deviation by a position gain is output from the position controller 100, an integrator of the speed controller 101 integrates (I controls in PI control), and the torque command is increased. If the torque command increases, the drive motor is driven in a state where output torque is increased (drive current is increased) and there is concern that the drive motor is brought into the excessive load state and this causes overheating.
The rotary table is driven by the drive motor which drives the rotary table, the rotary table is positioned at a predetermined positioned angle position and then, the rotary table is clamped by the clamping mechanism, and the positioned rotation position is held. Generally, this positioning carried out by the drive motor is determined as being completed when the drive motor reaches a location within a certain width of a commanded position (positioned position) (when the drive motor comes into in-position), and the rotary table is clamped by the clamping mechanism. At the time of this clamping operation, the rotary table is clamped in a state where it is rotated in some cases. There is caused a case where although the rotation position of the drive motor is in-position, the rotation position does not match with a target rotation position (positioned rotation position). As a result, position deviation is generated (if in-position width is set large, possibility that the position deviation becomes great is high). If a clamping force of the clamping mechanism is great, the position deviation is maintained, the torque command increases, output torque of the drive motor also increases, and large output torque is continuously output. As a result, the drive motor is brought into an excessive load state, temperature of the drive motor reaches overheating temperature and the machine tool is stopped in some cases.
To prevent the excessive load state and overheating of the drive motor, in JP 2008-134861 A, if a torque command value becomes equal to or higher than a permissible torque, control of a speed controller is switched from PI control to P control or a torque command is limited to a torque limit value or lower, and a drive motor is driven while suppressing output torque thereof to a small value to prevent the generation of the excessive load state. However, even if time is very short, the drive motor cannot output the output torque which is necessary to resolve the position deviation with respect to such a machining load that a torque command value exceeds permissible torque. Even when drive motor output torque which is limited to very short time where influence on heat generation of the drive motor is small is also limited to small torque, position deviation remains, and machining precision is deteriorated in a portion where large drive motor output torque is instantaneously required.